JP2012502993A - Binder-free adsorbents with improved mass transfer properties and their use in adsorptive separation of para-xylene - Google Patents

Abstract

Adsorbents and methods for the adsorptive separation of paraxylene from mixtures containing at least one other C8 aromatic hydrocarbon (eg, a mixture of orthoxylene, metaxylene, paraxylene, and ethylbenzene) are described. Suitable adsorbents include small crystallite size zeolite X with an average crystallite size of less than 1.8 microns. The adsorbent was free of binder (eg formulated in the substantial absence of an amorphous material that normally reduces selective pore volume) to further improve volume and mass transfer. May be. These properties are particularly advantageous for improving productivity in low temperature, low cycle time adsorption separation operations in simulated moving bed systems.
[Selection] Figure 6

Description

  The present invention relates to an adsorbent for adsorptive separation of para-xylene from a mixture containing at least one other C8 alkyl aromatic hydrocarbon (eg, a mixture of ortho-xylene, meta-xylene, para-xylene, and ethylbenzene). And methods. In particular, a binderless adsorbent comprising zeolite X with a small crystallite size has improved volumetric and mass transfer properties that favor the adsorptive separation process.

  In general, C8 alkyl aromatic hydrocarbons are considered to be valuable products and there is a high demand for paraxylene. In particular, the oxidation of para-xylene is used in the commercial synthesis of terephthalic acid, which is a raw material in the production of polyester fabrics. The primary source of para-xylene includes a mixed xylene stream that results from the refining of crude oil. Examples of such streams include the separation resulting from a commercial xylene isomerization process or a C8 alkyl aromatic hydrocarbon fraction resulting from a catalytic reformate by liquid-liquid extraction and fractional distillation. There is a flow, resulting from. Para-xylene may be separated by crystallization and / or adsorptive separation from a feed stream containing para-xylene that normally contains a mixture of all three xylene isomers. The latter technology accounts for the majority of the market share of newly constructed plants for the production of para-xylene.

  Thus, a number of patents have been directed to adsorptive separation from a feed stream containing a mixture of C8 alkyl aromatics of para-xylene. Zeolite X and Y have been used to selectively adsorb para-xylene. For example, US 3,686,342, US 3,903,187, US 4,313,015, US 4,899,017, US 5,171,922, US 5,177,295, US 5,495,061, and US 5,948,950 checking ... US Pat. No. 4,940,830 uses sodium zeolite Y or paraxylene from other xylene isomers and ethylbenzene using sodium zeolite Y that is also ion-exchanged with group IB or group VII elements. Rejective separation is described. A gas phase process by an adsorptive separation method for the recovery of para-xylene from a mixture of xylenes using an adsorbent comprising a crystalline molecular sieve having an average crystal size of 0.5 microns to 20 microns is disclosed internationally. No. 2008/033200.

  There remains a need in the art for improved adsorbents and processes for the efficient separation of para-xylene from relatively impure C8 alkyl aromatic hydrocarbon mixtures.

  The present invention relates to an adsorbent that selectively adsorbs paraxylene over at least one other C8 alkyl aromatic compound present in the mixture. Due to not only the evaporation (distillation) separation, but also due to practical limitations in reaction equilibrium / selectivity, typical mixtures obtained from the essential oil process (in addition to para-xylene) other xylene isomers I.e. ortho-xylene and meta-xylene in various amounts and usually also contains ethylbenzene. Usually such a mixture constitutes the feed stream used in the process associated with the present invention.

  Accordingly, embodiments of the present invention are directed to a process for separating para-xylene from a relatively impure mixture comprising one or more C8 alkyl aromatic hydrocarbons other than the desired para-xylene. This mixture is contacted with an adsorbent comprising zeolite X under adsorption conditions. The present invention provides “Small Crystallite Size Zeolite X” (ie, less than 1.8 microns, and typically, which can provide very favorable performance characteristics when incorporated into an adsorbent for use in adsorptive separation of para-xylene. In particular, it relates to the use of zeolite X) having an average crystallite size of 500 nanometers to 1.5 microns. In particular, (i) the mass transfer rate of paraxylene into the zeolite pores during adsorption, and (ii) the desorbent in the zeolite pores to replace the adsorbed paraxylene during desorption. The mass transfer rate is significantly greater compared to zeolite X synthesized according to conventional methods (and typically having an average crystallite size of 1.8 microns or greater).

  Accordingly, the adsorbents described herein are prepared from zeolite X with a small crystallite size, or crystallites such that at least a portion of the adsorbent is zeolite X having the average crystallite size described above. Zeolite X with a small size is included. The mass transfer characteristics of the adsorbents are typically such that zeolite X with a small crystallite size is in the adsorbent in an amount of at least 60 wt%, and often in an amount of 70 wt% to 90 wt%. The presence of zeolite X, which has a small crystallite size, is improved by being present. Low temperature operation (eg, below 175 ° C. (350 ° F.)) where mass transfer limitation associated with zeolite X-containing adsorbents having a conventional average zeolite X crystallite size becomes more commercially important In this case, an increase in the mass transfer rate is particularly advantageous. Increased paraxylene adsorption selectivity and adsorbent capacity, and increased liquid feed density, all directionally improve paraxylene productivity, but low temperature operation is desirable for many reasons, including these . However, in simulated moving bed operation, which is often used in continuous industrial processes for the adsorption separation of paraxylene from a feed mixture of orthoxylene, metaxylene, paraxylene, and ethylbenzene, lower operating temperatures It has been found that these benefits associated with are reduced as cycle time decreases due to mass transfer rate limiting which affects the rate of paraxylene adsorption / desorption. Thus, adsorbents with improved mass transfer properties can take advantage of these improvements in paraxylene capacity and selectivity as described above associated with lower temperature operation. Increased paraxylene productivity and resulting process economics can be improved.

  Another aspect of the invention relates to an adsorbent comprising zeolite X (eg, zeolite X with a small crystallite size as described above), which zeolite X is “binderless”. May be incorporated into the adsorbent, whereby a zeolite X precursor such as clay (eg kaolin clay), for example, is substantially converted to zeolite X, the conversion portion of which is a conventional crystallite size. (E.g., a crystallite size greater than 1.8 microns) but may itself be a small crystallite size zeolite or "nano-sized zeolite X" (ie, less than 500 nanometers) if desired And, typically, zeolite X) with an average crystallite size of 20 nanometers to 300 nanometers. By eliminating or substantially eliminating conventional binders (which usually only contribute to non-selective pore volume), (i) the desired extraction component (eg paraxylene) and / or (ii) ) The adsorbent capacity for the desorbent (eg paradiethylbenzene) can be significantly increased.

  The binder-free adsorbents described herein further have improved adsorption selectivity for the desired product paraxylene. Thus, both para-xylene / meta-xylene adsorption selectivity and para-xylene / ortho-xylene adsorption selectivity are increased compared to conventional bonded adsorbents. The desorbent strength remains maintained and paraxylene / ethylbenzene selectivity increases. These selectivity, capacity, and mass transfer benefits are comparable to conventional combined adsorbents under the same process conditions (ie, maintaining other operating parameters such as feed composition and process variables constant). When compared, it can translate to a 15-20% improvement in para-xylene productivity associated with the binder-free adsorbents described herein. In addition, the wear and strength properties of the binder-free adsorbent, as measured by water attrition analysis and piece crush strength analysis, are more physical than conventional adsorbents. Improvement in physical properties.

  In addition to kaolin clay, another zeolite X precursor is metakaolin clay, which results from activation of kaolin clay at high temperatures. Regardless of whether kaolin or metakaolin is used as the starting zeolite X precursor, this zeolite X precursor (which may be used first to bind a first portion containing zeolite X with a small crystallite size). A second (eg, conversion) containing zeolite X having a molar ratio of silica to alumina that is different from the molar ratio of silica to alumina in the first (eg, already prepared or prepared) portion. Part can be produced.

However, another aspect of the present invention is that the molar ratio of zeolite X second portion silica to alumina is greater than the molar ratio of zeolite X precursor to silica to alumina, and often zeolite. It is directed to the use of a silica source in the conversion of the zeolite X precursor to increase the value of the X first part silica to a molar ratio comparable to alumina. Thus, after conversion, a good example of the adsorbent resulting includes both _SiO 2 / _Al 2 _{O 3} molar ratio of 2.3 to 2.7, may contain a first portion and a second portion of zeolite X . The first portion may be a small crystallite size zeolite X having an average crystallite size of 500 nanometers to 1.4 microns, while the second portion is larger than 1.8 microns, conventional Zeolite X having a crystallite size of

  Accordingly, another aspect of the present invention relates to a method for preparing a binder-free adsorbent having improved mass transfer properties. The method forms particles comprising zeolite X having an average crystallite size of 500 nanometers to 1.5 microns (eg, zeolite X having a small crystallite size) and a zeolite X precursor (eg, kaolin clay); Activating the zeolite X precursor of the particles at a temperature of 500 ° C. to 700 ° C. (930 ° F. to 1300 ° F.) and digesting the particles containing the activated zeolite X precursor with a caustic solution Obtaining a binder-free adsorbent. This caustic digestion step is performed in the presence of a silica source (eg, sodium silicate or colloidal silica), and the molar ratio of silica to alumina in the converted portion of zeolite X in the adsorbent is determined as zeolite X. You may make it increase so that it may become larger than the molar ratio of the silica of a precursor with respect to an alumina. The use of a silica source and the corresponding increase in the molar ratio of converted zeolite X to alumina can result in further advantages, such as increased desorbent strength, which is This is advantageous for overall process performance in adsorptive separation.

Zeolite X with a small crystallite size used to formulate a binder-free adsorbent for adsorptive separation usually corresponds to a Si / Al ratio of 1.0 to 2.0 atoms, 2.0 A (SiO ₂ / Al ₂ O ₃ ) molar ratio of silica to alumina of molecules of ˜4.0. These ratios are usually applied not only to the first or “preparation” portion of zeolite X that is originally bound to the zeolite X precursor, such as, for example, metakaolinite, but also result from the conversion of the zeolite X precursor. This also applies to “transformed” zeolite X. However, as indicated above, the non-binder-containing zeolite X results from the different types of zeolite X in these parts of the final adsorbent (ie, the preparation part and / or the conversion part), and As a result of the addition of the silica source during the conversion of the zeolite X precursor to increase the molar ratio of silica to alumina in the conversion portion, it may have portions where the molar ratio of silica to alumina is slightly different. .

  The zeolite, whether it is a preparation part or a conversion part, usually has at least 95% and typically substantially all (at least 99%) of its ion exchange exchanged by barium or a combination of barium and potassium. Has a possible site. Typical adsorbents have a small crystallite size with 60% to 100% of their ion-exchangeable sites exchanged by barium and 0% to 40% of their ion-exchangeable sites exchanged by potassium Contains zeolite X.

  The binder-free adsorbent comprising zeolite X with a small crystallite size as described above is used for solid adsorbents used in fixed bed, moving bed, or simulated moving bed adsorption separation processes using conventional adsorption conditions. be able to. Adsorption may be carried out in the liquid phase or in the gas phase, and usually liquid phase adsorption conditions are preferred. When used for adsorption separation of para-xylene in a simulated moving bed system, conventional adsorption operating at the same percentage overall para-xylene recovery due to the high adsorbent capacity / mass transfer characteristics of the adsorbent described above. Compared to the agent, para-xylene productivity can be relatively increased, especially in the case of low cycle time operation. That is, the adsorbent bed concentration profile is not adversely affected when the cycle time is, for example, less than 34 minutes (eg, in the range of 24 minutes to 34 minutes). The cycle time of the simulated moving bed adsorption separation process refers to the time at which either the inlet or outlet flow returns to its initial adsorbent bed position. Thus, in a typical simulated moving bed mode of operation using 24 adsorbent beds (eg, two containers each having 12 beds), cycle time is, for example, zero time to the first bed. And the time required for the inlet feed stream introduced first to be reintroduced into the bed. Shorter cycle times translate into higher productivity when all other factors (eg, paraxylene purity and recovery) are equal.

  Thus, certain embodiments of the present invention separate para-xylene from a mixture containing at least one other C8 alkyl aromatic hydrocarbon, usually containing the xylene isomers ortho-xylene and meta-xylene and ethylbenzene. Related to the process. The process includes contacting the mixture with a binder-free adsorbent comprising at least a portion of zeolite X having a small crystallite size having an average zeolite crystallite size in the range described above. Exemplary adsorption temperatures range from 60 ° C. (140 ° F.) to 250 ° C. (480 ° F.). However, due to their improved capacity and mass transfer properties, these adsorbents do not produce appreciable mass transfer rate limitations associated with conventional adsorbents when operating at low temperatures. Thus, as explained above, the benefits associated with improved adsorption paraxylene selectivity and adsorbent capacity at relatively low temperatures can be more fully realized. Adsorption temperatures below 175 ° C. (350 ° F.), such as 130 ° C. (270 ° F.) to 165 ° C. (330 ° F.) are particularly advantageous when used with the adsorbents described above. The adsorption pressure may be slightly above normal pressure, for example in the range of 1 barg (15 psig) to 40 barg (580 psig).

  Contacting a mixture of C8 alkyl aromatic hydrocarbons as described above (eg, as a continuous or batch process feed) with a binder-free adsorbent takes precedence over at least one other C8 alkyl aromatic hydrocarbon. And usually, in preference to all such hydrocarbons present in the mixture, result in or cause the adsorption of para-xylene into zeolite X pores with a small crystallite size. Thus, the adsorbed phase (ie within the zeolite X pores) is selectively enriched in paraxylene content compared to the paraxylene content of a mixture of C8 alkyl aromatics (eg, feed stream). If the mixture contains ortho-xylene, meta-xylene, para-xylene, and ethylbenzene, para-xylene will be present in the adsorbed phase in an abundant amount compared to the mixture, and ortho-xylene , Metaxylene, and ethylbenzene will be present in the non-adsorbed phase in abundant amounts compared to the mixture.

  This non-adsorbed phase may then be removed (or washed away) from contact with the adsorbent, for example in a raffinate stream. This adsorbed phase enriched in para-xylene may be desorbed away from the adsorbent, for example in an extraction stream. A desorbent stream comprising a desorbent, for example an aromatic ring-containing compound such as toluene, benzene, indane, paradiethylbenzene, 1,4-diisopropylbenzene, or mixtures thereof, can be used for both the flushing and desorption. . The adsorption separation process using the adsorbent described above may be performed by a simulated moving bed system. According to this embodiment, the C8 alkyl aromatic hydrocarbon feed stream and the desorbent stream described above are loaded into a fixed bed of the adsorbent while the extract stream and raffinate stream are removed from the bed. Is done. These streams may be filled and removed continuously.

  During the simulated moving bed system or other type of adsorption separation system, monitor the water content of the outlet stream, such as the extract stream or raffinate stream, to determine the water content or hydration level of the adsorbent. May be desirable. If necessary, water is added either continuously or intermittently to an inlet stream such as, for example, a feed stream and / or a desorbent stream to achieve a desired adsorbent hydration level (eg, 4 % To 7% of loss on ignition (corresponding to Loss in Ignition)) may be maintained. Alternatively, water is added to correspond to this adsorbent hydration level or to another desired adsorbent hydration level, for example from 20 ppm to 120 ppm by weight of the extract stream and / or raffinate stream. Absolute water content may be obtained.

  These and other aspects and features related to the present invention will be apparent from the following detailed description.

FIG. 1 shows the effect on strength of paradiethylbenzene desorbent by reducing the molar ratio of barium exchanged zeolite X to silica to alumina from 2.5 to 2.0. FIG. 2 shows the selectivity of the feed mixture components as a function of temperature (ie, para-xylene / meta-xylene selectivity “P / M”) obtained from a pulse test using an adsorbent containing zeolite X; / Ortho-xylene selectivity “P / O”; and para-xylene / ethylbenzene selectivity “P / E”). FIG. 3 shows the volume of adsorbent containing zeolite X as a function of temperature, as well as paraxylene / paradiethylbenzene selectivity “PX / pDEB Sel”, obtained from breakthrough (or dynamic) testing. FIG. 4 shows “DW” or “Delta W” as a function of temperature, ie the half-width of the para-xylene peak (ie, at half strength), as a function of temperature, obtained from a pulse test using an adsorbent containing zeolite X. It shows the value obtained by subtracting the half width of the linear nonane (n-C9) tracer peak from the peak envelope width. FIG. 5 shows the performance of a paraxylene adsorption separation process operating in a simulated moving bed mode at temperatures of 150 ° C. (302 ° F.) and 177 ° C. (350 ° F.). FIG. 6 shows the cycle time in a paraxylene adsorption separation process operating in a simulated moving bed mode for a conventional adsorbent containing zeolite X crystallites and another adsorbent containing zeolite X with reduced crystallite size. The impact on the recovery rate of para-xylene is shown. FIG. 7 shows the size distribution of both conventional zeolite X crystallites and zeolite X crystallites having a reduced size. FIG. 8 shows the loss on ignition (LOI) function measured for binder-free adsorbents using a pulse test at temperatures of 150 ° C. (302 ° F.) and 177 ° C. (350 ° F.). Comparison of paraxylene / metaxylene selectivity “P / M”; paraxylene / orthoxylene selectivity “P / O”; and paraxylene / ethylbenzene selectivity “P / E” as It shows with P / E (P / E dyn) measured using the breakthrough test. FIG. 9 shows a comparison of adsorbent capacity for binder-free adsorbents measured using a pulse test at temperatures of 150 ° C. (302 ° F.) and 177 ° C. (350 ° F.).

  The present invention relates to the separation of para-xylene from a mixture comprising at least one other C8 alkyl aromatic hydrocarbon. The term separation refers to the recovery of paraxylene in a stream (eg, product stream) or fraction enriched in paraxylene content (ie, having a higher content than originally present in the mixture). . The separation comprises binding the mixture comprising at least a portion of zeolite X with a small crystallite size having an average crystallite size of less than 1.8 microns and typically between 500 nanometers and 1.5 microns. This is achieved by contacting with a non-containing adsorbent. As noted above, the use of a binder-free adsorbent improves the selective micropore volume for para-xylene and other C8 alkyl aromatics, thereby allowing increased production rates for a given operation. Also, the use of zeolite X with a small crystallite size can overcome mass transfer rate limiting and fully exploit the advantages of a thermodynamically favorable operating regime (eg, low temperature and low cycle time). it can.

The structure of zeolite X is described in detail in US 2,882,244. Zeolite X with a small crystallite size can be prepared by seeded synthesis. In seed crystal synthesis, a seed or starting material that is used as a means of nucleation or starting zeolite crystallite growth is first prepared and then into the gel composition corresponding to the target crystallite size. Gel composition: blended in a seed crystal ratio. The ratio of the gel composition to the seed crystal determines the relative number or concentration of nucleation sites, which also affects the crystallite size of the synthesized zeolite X. The crystallite size decreases directionally if the seed crystals are in larger quantities or higher concentrations. For example, the preparation of zeolite X having an average crystallite size of 2 microns and 0.5 microns can be made using gel: seed ratios of 5400: 1 and 85: 1, respectively, by weight. In view of this disclosure, one of ordinary skill in the art can easily vary this weight ratio to achieve other average crystallite sizes. A typical gel composition includes Na ₂ O, SiO ₂ , Al ₂ O ₃ , and water. In this gel, 1 to 5 moles of Na ₂ O and SiO ₂ and 100 to 500 moles of water can be used for 1 mole of Al ₂ O ₃ .

  The gel composition can be prepared by mixing a gel makeup solution with, for example, an aluminate makeup solution containing 12% alumina by weight. The gel-making solution is prepared by mixing water, caustic solution, and sodium silicate and cooling the mixture to 38 ° C. (100 ° F.). The aluminate preparation solution can be prepared by dissolving alumina trihydrate in a caustic solution while heating as necessary for dissolution, then cooling and aging at 38 ° C. (100 ° F.). This is then mixed with the gel making solution. The gel-making solution and aluminate solution are then mixed for a short time (eg, 30 minutes) with vigorous stirring before the required amount of seed crystals is added.

The seed crystal is prepared by the same method as that for the gel composition. Thus, a typical seed crystal composition also includes Na ₂ O, SiO ₂ , Al ₂ O ₃ , and water. With respect to 1 mol of Al ₂ O ₃ , 10 to 20 mol of Na ₂ O and SiO ₂ and 150 to 500 mol of water can be used. The aluminate solution used in the preparation of the seed crystals may contain, for example, 18% alumina by weight. After mixing the gel composition and seed crystals, the mixture is heated with continued stirring and then at a temperature of 25 ° C. (75 ° F.) to 150 ° C. (300 ° F.) under stirring conditions. In order to achieve the desired crystallite formation from the seed crystal nuclei. The resulting solid material can then be filtered, washed and dried to obtain a prepared zeolite X with a small crystallite size.

  Zeolite X with a small crystallite size can then be used for the synthesis of binder-free adsorbents by mixing this “prepared” or already made part with the zeolite X precursor. Zeolite X precursors include, for example, clays such as kaolin, kaolinite, and halloysite, and other minerals such as hydrotalcite, and, for example, precipitated and fumed amorphous silica, precipitated alumina, gibbite, boehmite, bayer. Light and solid silica and alumina sources such as transition aluminas such as gamma and eta alumina, and zeolite seed solutions and suspensions obtained from sodium silicate and sodium aluminate, and similar reagents. These can be formed in an intimate mixture containing crystallites in the preparation portion of zeolite X with a small crystallite size. The formation procedure involves the addition of a zeolite X precursor, eg kaolin clay, zeolite X powder with a small crystallite size, and optionally other pore-forming materials (eg corn starch to give microporosity) Mixing with the agent as well as the water necessary to obtain a suitable hardness for molding. Larger particles (eg, US Standard mesh sizes in the range of 16-60 can be prepared. The formed particles containing the zeolite X having a small crystallite size and the zeolite X precursor are then activated, usually at a temperature in the range of 500 ° C. to 700 ° C. (930 ° F. to 1300 ° F.). In the case of zeolite X precursors containing kaolin clay, activation causes the endothermic dehydroxylation of this material, thereby forming a disordered metakaolin phase. After activation, the activated zeolite X precursor is converted to zeolite X itself by caustic digestion of the formed particles (eg, using sodium hydroxide), resulting in binder-free adsorption. An agent is produced. The binder-free adsorbent may comprise or consist essentially of zeolite X having a small crystallite size, or zeolite X having a small crystallite size together with a conversion portion of zeolite X. . Here, the converted portion of the zeolite X may have a conventional average crystallite size (eg, a size average crystallite size greater than 1.8 microns) or, if desired, “nano-sized zeolite X” (ie, Zeolite X) having an average crystallite size of less than 500 nanometers, and typically 20 nanometers to 300 nanometers. Photomicrographs of the binder-free adsorbent by high resolution scanning electron microscopy (HR SEM) showed that the zeolite X precursor was converted to nanosized zeolite X.

The molar ratio of the converted portion of zeolite X to the silica to alumina, and the contribution of this material in the final adsorbent formulation, will vary depending on the type and amount of zeolite X precursor incorporated into the formed particles. Also good. Usually, the ratio of the zeolite X precursor to silica to alumina is kept substantially constant during the conversion to zeolite X. Thus, a typical kaolin clay having a _SiO 2 / _Al 2 _{O 3} molar ratio of 2.0 to 2.2 is turned into zeolite X portion having a zeolite framework (the zeolite framework) ratio within this range. Thus, it is possible to prepare binder-free adsorbents having a first (preparation) portion and a second (conversion) portion of zeolite X in various silica to alumina ratios.

  However, an aspect of the present invention is that in the adsorption separation of paraxylene by increasing the ratio of zeolite X to silica to alumina from the range of 2.0-2.2 to the range of 2.3-2.7 (eg Related to the discovery that the strength of the desorbent (using paradiethylbenzene desorbent) is increased. This is illustrated in FIG. 1, where FIG. 1 shows the paradiethylbenzene desorption by reducing the molar ratio of barium exchanged zeolite X to silica from 2.5 to 2.0. This shows the influence on the strength of the agent. The line in FIG. 1 represents a pulse and breakthrough (or dynamic) test, described in more detail below, of an adsorbent comprising barium exchanged zeolite X having a silica to alumina molar ratio of 2.5. Created from the data used. In particular, this line shows the relationship between the reciprocal of the relative desorbent strength and the adsorbent bed temperature.

  When zeolite X having a silica to alumina molar ratio of 2.0 is replaced with a higher ratio of zeolite X in the adsorbent formulation, the strength of the paradiethylbenzene desorbent is that of the lower ratio in FIG. Significantly decreases (ie, increases the reciprocal value of the relative desorbent strength), as shown by the data points (triangles) obtained for zeolite X. This reduction in desorbent strength affects the ability of the desorbent to move the desired paraxylene product into the extract stream, and is particularly adsorbent separation of paraxylene operating in a simulated moving bed mode. In the commercial process for the production of para-xylene in terms of obtaining high purity and high recovery rate can be detrimental.

  These experimental results show that the molar ratio of silica to alumina in which the conversion part of the zeolite X is in the range of 2.1 to 2.7 (this is the same or substantially the same as the molar ratio of silica to alumina in the preparation part of the zeolite X). The process efficiencies that can be obtained by the use of a binder-free adsorbent having (which may be identical to each other). However, zeolite X precursors, such as kaolin clay, for example, often have a lower, for example 2.0, silica to alumina molar ratio, and thus usually the desired higher ratio of zeolite X does not change.

In the synthesis of the binder-free adsorbent, the procedure for converting the zeolite X precursor to zeolite X can be modified to increase the molar ratio of silica to alumina in the converted portion of the zeolite X. I understood. In particular, this is accomplished by the addition of a silica source such as, for example, colloidal silica sol, silicic acid, sodium silicate, silica gel, or reactive particulate silica (eg, diatomaceous earth, Hi-Sil, etc.). The silica source may be added during the adsorbent particle formation step, or may be added to the caustic digestion step, or both. The amount of silica added causes the overall reaction mixture containing the zeolite X precursor (eg, metakaolin) and the silica source to be adjusted so that the reaction composition falls within the following range: Na ₂ O / SiO _{2 = 0.8~1.5, SiO 2 / Al 2} O 3 = 2.5~5, H 2 O / Na 2 O = 25~60.

  Thus, by using another source of silica, the molar ratio of silica to alumina in both the preparation part and the conversion part of zeolite X was approximately matched (for example, both 2.0-3.0, and Can be used to prepare binder-free adsorbents, which are usually in the range of 2.1 to 2.7, thereby using a lower ratio of zeolite X in the adsorption separation of para-xylene The above disadvantages can be overcome. Conveniently, the increased hydrothermal stability of the resulting binder-free adsorbent may be improved by increasing the molar ratio of silica to alumina in the conversion portion of zeolite X.

  The relative amounts of zeolite X preparation and conversion moieties in the binder-free adsorbent may vary. According to some embodiments, the amount of zeolite X precursor used to prepare the formed particles ranges from 5 wt% to 40 wt%, and usually from 10 wt% to 30 wt%. Therefore, these ranges also correspond to the amount of converted zeolite X present in the representative binder-free adsorbent described herein.

  Various other embodiments are possible when aiming to obtain a binder-free adsorbent having at least a portion of zeolite X with a small crystallite size. For example, a zeolite X portion having a small crystallite size may be produced as a preparation portion synthesized as described above, or a zeolite X portion having a small crystallite size may be obtained by conversion of a zeolite X precursor. It may be the resulting conversion part. In this case, the preparation portion of the zeolite X may have a small crystallite size or a larger average crystallite size (eg, an average crystallite size greater than 1.8 microns, which is characteristic of conventional zeolite X) This is because in any embodiment, the binder-free adsorbent will have at least a portion of zeolite X with a small crystallite size. Typically, the adsorbents described herein have a small crystallite size, whether in the preparation part, in the conversion part, or in a combination of both parts. A portion of zeolite X is included in a significant amount. Thus, the adsorbent typically comprises zeolite X with a small crystallite size in an amount of at least 60% by weight, and often has a small crystallite size in an amount of 70% to 90% by weight. Contains zeolite X.

  If the zeolite X precursor is converted to zeolite X with a small crystallite size, the binder-free adsorbent may preferentially contain this converted zeolite X, or else the adsorption The agent may entirely comprise a converted portion of zeolite X with a small crystallite size, or may consist essentially of a converted portion of zeolite X with a small crystallite size. In order to synthesize an adsorbent containing the converted zeolite X in whole or substantially entirely, the zeolite X precursor is molded or formed into larger beads, spheres, pellets, etc. using the conventional methods described above, Larger particles (e.g., in the range of US standard mesh size 16-60) may be prepared, but the addition of the zeolite X preparation portion is not performed.

  According to other embodiments, nanosized zeolite X (i.e., zeolite X having an average crystallite size of less than 500 nanometers and typically 20 nanometers to 300 nanometers) is added to the zeolite X precursor. It can be obtained from the body as a conversion part. For the above-described embodiments where the conversion portion of zeolite X is zeolite X with a small crystallite size or nano-size zeolite X, the types of zeolite X precursors that can be converted to zeolite X with these altered crystallite sizes As sodium silicate, sodium aluminate, aluminum sulfate, aluminum nitrate, aluminum chlorohydrate, and / or fumed, precipitated, colloidal, organometallic, or other solid or liquid silicon ( Or a source of silica) or aluminum (or alumina), but is not limited thereto. Such other sources include clays and related minerals, and those resulting from combinations of two or more of the precursors described above to form aluminosilicate precursors, such as amorphous aluminosilicates Salt co-precipitates, sol-gels, or other solids formed from them. Zeolite X crystallite size depends on the reaction pH, time, temperature, degree of agitation, and concentration of reagents (including those mentioned above and others such as sodium hydroxide or other mineralizers). The size is adjusted within the range described above for small zeolite X or nano-sized zeolite X. For example, the zeolite X crystallite size of the conversion portion can be changed by changing the particle size of the zeolite X precursor such as kaolin or halloysite clay.

  Thus, the binder-free adsorbent described herein may comprise a preparation portion of zeolite X and a conversion portion of zeolite X. The preparation part and the conversion part may be prepared according to the above-described procedure. For example, the preparation part may be prepared using the synthesis procedure of zeolite X having a small crystallite size, and the conversion described above. The conversion portion may be made using a procedure. These adsorbents can then be used for the adsorption separation of para-xylene according to the procedures described herein. Those of ordinary skill in the art and viewing this disclosure will be aware that the molar ratio of silica to alumina, average crystallite size, relative amount, and other properties for the preparation and conversion portions of zeolite X are determined for a given application. It will be appreciated that the desired qualities of the resulting adsorbent, including strength, ion exchange capacity, adsorbent capacity, desorbent selectivity, mass transfer, etc., will depend on

  In typical binder-free adsorbents, there is substantially no non-zeolitic material (eg, typically less than 2 wt%, typically less than 1 wt%, and often in the adsorbent) Present in an amount of less than 0.5% by weight). The absence or substantial absence of non-zeolitic or amorphous materials can be confirmed using X-ray diffraction and / or high resolution scanning electron microscopy (HR-SEM) to verify the crystal structure. This can be confirmed by analyzing the binder-free adsorbent. The structure and distribution of macropores and micropores can be characterized and confirmed using mercury porosimetry or liquid oxygen adsorption.

  The portion of the prepared zeolite X (eg, zeolite X with a small crystallite size) and converted zeolite X in the binder-free adsorbent may initially be in the sodium form, and the sodium cation is known in the art. May be partially or fully exchanged with another cation such as barium, potassium, strontium, and / or calcium. For example, a synthetic binder-free adsorbent in which zeolite X has at least some of its ion-exchangeable sites in the form of sodium ions can be obtained by ion exchange of sodium ions with barium ions and / or potassium ions or It may be immersed in a barium ion-containing solution or in a barium and potassium ion-containing solution at conditions of temperature and time that can result in substitution (eg, at 20-125 ° C. for 0.5-10 hours). By column operation according to known techniques, for example, by feeding a preheated barium chloride / potassium chloride solution into the column of adsorbent particles, ion exchange is performed to completely replace the sodium cation of zeolite X. Also good. Filter the binder-free adsorbent, remove it from the solution, and re-immerse it in a new solution (eg, with the same or different ratio of cations or other types of cations) to the desired type and ratio. Repeat until the desired exchange level is achieved for the cations. Typically, at least 95% or substantially all of the binder-free adsorbent containing zeolite X with a small crystallite size as either a preparation or conversion part of zeolite X or a combination of both parts (e.g., at least 99%) of zeolite X exchanged by barium or a combination of barium and potassium. Usually, other metal ions can exchange ions in the preparation part of zeolite X (eg zeolite X with a small crystallite size) or conversion part of zeolite X in an amount effective to change the adsorption properties of the adsorbent. It does not occupy a part. In an exemplary embodiment, zeolite X with a small crystallite size comprises 60% to 100% ion exchangeable sites exchanged by barium and 0% to 40% ion exchangeable sites exchanged by potassium. Have

The number of ion-exchangeable sites decreases as the overall SiO ₂ / Al ₂ O ₃ molar ratio of the zeolite X increases. This overall ratio can be influenced by changing the ratio of one or both of the prepared zeolite X portion and the converted zeolite X portion. Also, the total number of cations per unit cell decreases as monovalent cations (eg, K ⁺ ) are replaced by divalent cations (eg, Ba ⁺² ). There are many ion exchangeable sites in the zeolite X crystal structure, some of which are located outside the super cage. Overall, the number and location of cations in the zeolite crystal structure depends on the size and number of cations present and the SiO ₂ / Al ₂ O ₃ molar ratio of the zeolite.

  Those skilled in the art will recognize that adsorbent performance (eg, with respect to purity and recovery of para-xylene in the extract stream) depends on many conditions such as operating conditions, feed stream composition, water content, and desorbent type and composition. You will understand that it is affected by process variables. Thus, the optimal desorbent composition depends on many interrelated variables. Typically, the overall paraxylene recovery from the feed stream is achieved by a process for adsorptive separation of paraxylene from a mixture containing at least one other C8 alkyl aromatic hydrocarbon as described herein. In a high state (eg, at least 90%, 92% -99.5%, or 95% -99%) and high paraxylene purity (eg, at least 99% or at least 99.5% by weight) in the extraction product stream. Even) can be achieved.

  One issue to consider regarding overall adsorbent performance is its water content, which is sufficient to reach a constant weight of an unused adsorbent sample at 900 ° C. under an inert gas purge such as nitrogen. It can be determined by the Loss on Ignition (LOI) test, which measures the weight difference obtained by drying over a period of time (eg 2 hours). The sample is first conditioned at 350 ° C. for 2 hours. The weight loss rate based on the initial weight of the sample is LOI at 900 ° C. For binderless adsorbents containing zeolite X with a small crystallite size, a LOI of 4% to 7% by weight, and preferably 4% to 6.5% by weight, is usually desirable. An increase in LOI value can be achieved by equilibrating a certain amount of adsorbent with a lower LOI than desired and a certain amount of water in a closed desiccator until equilibrium is established. The LOI value of the adsorbent may be reduced by drying at 200-350 ° C. under an inert gas purge or under vacuum for a period of time (eg 2 hours).

  In order to monitor the adsorbent LOI during the adsorptive separation process, it may be desirable to measure the water content of such an appropriate outlet stream, such as a raffinate stream and / or an extract stream. Also, the adsorbent LOI may be adjusted or maintained, if desired, by addition or injection of continuous or intermittent water into the appropriate inlet stream, for example into the feed and / or desorbent stream. can do. According to one embodiment, the adsorbent LOI is maintained by monitoring the water content in the extract stream and / or raffinate stream. For example, a typical range of water contained in one or both of these streams corresponding to the desired adsorbent LOI is 20 ppm to 120 ppm by weight. Often, the desired water content contained in one or both of these outlet streams is between 40 ppm by weight and 80 ppm by weight. Water is added to the feed stream and / or desorbent stream as described above by continuous or intermittent injection to maintain these measured water levels contained in the extract stream, raffinate stream, or both. can do.

  During this analysis, other volatile components (eg, organic materials), typically on the order of 0.5% by weight, are also lost, so LOI is an indirect measure of adsorbent hydration level (or water content) or It is understood that it is a relative measurement. Therefore, the desired adsorbent water content is simply the water content corresponding to a LOI of 4 wt% to 7 wt% measured as described above. If necessary, the absolute amount of water in the zeolitic adsorbent sample can be determined by known analytical methods such as Karl Fischer (ASTM D 1364).

  The adsorption separation of para-xylene from other C8 alkyl aromatic compounds may be carried out either in the liquid phase or in the vapor phase, but usually the liquid phase operation is predominantly used. As mentioned above, the adsorption temperature below 175 ° C. (350 ° F.), for example 130 ° C. (270 ° F.) to 165 ° C. (330 ° F.), is a binder-free adsorption comprising zeolite X with a small crystallite size It is particularly advantageous when used in preparations. This is because at these temperatures, mass transfer rate limiting, which prevents conventional adsorbents from exploiting improved paraxylene adsorption selectivity and adsorbent capacity, is overcome. Adsorption conditions may include pressures ranging from normal pressure to 600 psig, with pressures typically from 1 barg (15 psig) to 40 barg (580 psig). Desorption conditions often include substantially the same temperature and pressure used for adsorption.

  As described above, the separation of para-xylene is performed by bringing a mixture of para-xylene and at least one other C8 alkyl aromatic hydrocarbon into contact with an adsorbent under adsorption conditions. For example, a feed stream containing a mixture of C8 alkyl aromatic hydrocarbons is contacted with the bed of adsorbent to selectively adsorb paraxylene in the adsorption phase over orthoxylene, metaxylene, and ethylbenzene. Can be made. These other C8 alkyl aromatic components of the feed stream can selectively pass through the adsorption zone as a non-adsorbing phase.

  A feed stream comprising a mixture of C8 alkyl aromatic hydrocarbons can be separated from various refining process streams (eg, reformate), including the product of the separation unit. Such separation is inaccurate and therefore the feedstock is limited to other compounds such as C9 alkyl aromatic carbonization (eg, less than 5 mol%, and often less than 2 mol%). It is expected to contain hydrogen. In most cases, the feedstock is predominantly C8 alkyl aromatic hydrocarbons and totals less than 10 mol%, typically less than 5 mol%, and sometimes less than 1 mol%, Contains other types of compounds. In one type of separation process, after reaching the adsorption capacity of the adsorbent, the feed inlet flow to the adsorbent is stopped, and then the adsorption zone is washed away to remove the non-adsorbed phase that originally surrounded the adsorbent. Remove from contact with the adsorbent. The adsorbed phase enriched in the desired paraxylene is then used to adsorb the adsorbent with a cyclic hydrocarbon such as toluene, benzene, indane, paradiethylbenzene, 1,4-diisopropylbenzene, or mixtures thereof (eg, aromatic ring containing carbon It can be desorbed from the adsorbent pores by treatment with a desorbent that normally contains hydrogen). In general, (i) washing the non-adsorbed phase into a raffinate stream containing the desorbent, and (ii) desorbing the adsorbed phase into an extract stream that also contains the desorbent, The same desorbent is used for both. Since this extract stream contains an adsorbed phase enriched in para-xylene, this extract stream is also considered when considered on the basis of no desorbent compared to the feed stream. Rich in para-xylene.

  As used herein, a “feed stream” is a mixture containing the desired extraction component (paraxylene) and one or more raffinate components to be separated by the adsorptive separation process. Thus, the “feedstock mixture” (ie, including “feedstock mixture components”) includes extraction components and raffinate components such as, for example, a mixture of xylene (orthoxylene, metaxylene, and paraxylene described above) and ethylbenzene A mixture is included. The feed stream is the inlet stream to the adsorbent (eg in the form of one or more adsorbent beds) used in the process. An “extracted component” is a compound or type of compound that is selectively adsorbed by an adsorbent. A “raffinate component” is a compound or type of compound that is less selectively adsorbed (or selectively excluded). A “desorbent” is typically any material capable of desorbing an extracted component from an adsorbent, and a “desorbent stream” is an inlet stream to an adsorbent that contains a desorbent. A “raffinate stream” is the outlet stream from the adsorbent, where the raffinate component is removed. The composition of the raffinate stream can vary from essentially 100% desorbent to essentially 100% raffinate components, with a small amount being one or more extraction components. An “extraction stream” is the exit stream from the adsorbent, where the extracted components are removed. The composition of the extract stream may vary from essentially 100% desorbent to essentially 100% extract components, with a minor amount being one or more raffinate components.

  Typically, at least a portion of the extract stream and raffinate stream is purified (eg, by distillation) to remove the desorbent, thereby producing an extract product stream and a raffinate product stream. The “extracted product stream” and “raffinate product stream” are respectively present in the feed stream at a higher concentration and in the extract stream than in the extract stream and raffinate stream, respectively. Refers to the product produced by the adsorptive separation process, contained at a higher concentration.

  Increasing the volume can reduce the amount of adsorbent (and consequently cost) required to separate the extractables for a specific feed rate of the feed mixture so that a specific volume of The capacity of the adsorbent for adsorbing the extracted components is an important property. A good initial capacity for the extracted components as well as the total adsorbent capacity should be maintained over some economically desirable lifetime during actual use in the adsorption separation process.

  The rate of exchange of the extracted component (paraxylene) by the desorbent is typically at half the strength obtained by plotting the composition of the various species in the adsorption zone eluate obtained during the pulse test against time. Can be characterized by peak envelope width. The narrower the peak width, the faster the desorption rate. Desorption rate can also be characterized by the distance between the center of the tracer peak envelope and the disappearance of the extracted components that have just been desorbed. This distance is time dependent and is therefore a measure of the volume of desorbent used during this time interval. The tracer is usually a relatively non-adsorbing compound that moves faster through the adsorbent column than the material to be separated.

The selectivity for the raffinate component for the extract component (β) is the distance between the center of the extract component peak envelope and the tracer peak envelope (or other reference point), the center of the raffinate component peak envelope and the tracer peak envelope (or Ratio to the corresponding distance between the reference points). The selectivity corresponds to the ratio of the two components in the adsorbed phase divided by the ratio of the same two components in the non-adsorbed phase at equilibrium conditions. Therefore, selectivity can be calculated from the following equation:
Selectivity = (wt% C _A / wt% D _A ) / (wt% C _U / wt% D _U )
Where C and D represent the two components of the feed mixture in weight percent, and the subscripts A and U represent the adsorbed phase and the non-adsorbed phase, respectively. Equilibrium conditions are determined when the feedstock passing over the adsorbent bed does not change composition, in other words, when there is no net movement of material between the non-adsorbed and adsorbed phases. In the above equation, a selectivity greater than 1.0 indicates that component C is preferentially adsorbed within the adsorbent. Conversely, a selectivity of less than 1.0 indicates that component D is preferentially adsorbed, so that the non-adsorbed phase is richer in component C and the adsorbed phase is richer in component D. Will.

  If the selectivity of the two components approaches 1.0, the adsorbent does not preferentially adsorb one component to the other (ie, they are both To the same extent). As selectivity deviates from 1.0, one component is more and more preferentially adsorbed by the adsorbent over the other. Selectivity can be expressed not only for one feedstream compound relative to one feedstream compound (eg, selectivity for paraxylene over metaxylene) but also between any feedstream compound and desorbent. (Eg, selectivity of para-xylene over para-diethylbenzene).

  In the case where the adsorbent selectivity for the raffinate component of the extract component is only slightly greater than 1, separation of the extract component from the raffinate component is theoretically possible, but considering the process economy, this selectivity is Preferably it is at least 2. Similar to the relative volatility in fractional distillation, the higher the selectivity, the easier it is to perform adsorption separation. By increasing the selectivity, the amount of the adsorbent to be used can be reduced in a directional manner. This is just as the higher relative volatility requires fewer theoretical distillation steps (and smaller columns) to perform a given distillation separation for a given feedstock. .

  The desorbent for the adsorptive separation process must be wisely selected to meet several criteria. Ideally, the desorbent is not adsorbed so strongly as to prevent the extractant from replacing the desorbent in the subsequent adsorption cycle, but replaces the extractant from the adsorbent at a reasonable mass flow rate. It should have sufficient strength (ie, be adsorbed sufficiently strongly). From the point of view of selectivity, the adsorbent is preferably more selective for the extraction component for the raffinate component than for the desorption agent for the raffinate component. In addition, the desorbent must be compatible with both the adsorbent and the feed mixture. In particular, the desorbent must not adversely affect the desired selectivity of the adsorbent for the raffinate component of the extracted component. In addition, since the extract stream and raffinate stream are typically both removed from the adsorbent in the mixture by the desorbent, the desorbent is essentially chemically inert to the extract and raffinate components. Should. Any chemical reaction involving the desorbent and the extract or raffinate component will complicate or possibly prevent product recovery.

  Therefore, the performance parameter to be considered for the desorbent is its rate of exchange of the extracted components of the feedstock, in other words the relative rate of desorption of the extracted components. This parameter is directly related to the amount of desorbent that must be used in the adsorption separation process to desorb the extracted components from the adsorbent. The faster exchange rate reduces the amount of desorbent required and, therefore, operating costs associated with process streams containing larger desorbents (including separation and recycling of desorbents from these streams). ) Decreases. A slightly lower desorbent selectivity for the extract component of 1 or slightly helps to ensure that all extract components are desorbed by a desorbent at a reasonable flow rate, and in subsequent adsorption steps, It also helps to ensure that the extracted components are replaced with the desorbent.

  Further, since both the raffinate stream and the extract stream usually contain a desorbent, the desorbent should also be easily separable from the mixture of extractive and raffinate components introduced into the feed stream. Without a method for separating the desorbent in the extract stream and raffinate stream, the concentration of the extract component in the extract product and the concentration of the raffinate component in the raffinate product would not be commercially preferred and The desorbent will not be available for reuse in the process. Thus, other separation methods such as reverse osmosis can be used alone or in combination with distillation or evaporation, but usually at least a portion of the desorbent is extracted from the adsorption separation process. Recovered from the stream and raffinate stream by distillation or evaporation. In this regard, the desorbents are typically “light” or “heavy” desorbents, which are distilled from C8 alkyl aromatic hydrocarbons in the extraction and raffinate streams of the paraxylene adsorption separation process, respectively. It can be easily separated as an overhead product or a bottoms product.

  A “pulse test” can be used to test an adsorbent with a particular feed mixture and desorbent to determine adsorbent properties such as adsorption capacity, selectivity, resolution, and exchange rate. A typical pulse test apparatus is a tubular adsorbent chamber having a volume of 70 cubic centimeters (cc) having an inlet portion and an outlet portion at both ends of the chamber. This chamber is provided to allow operation at a constant, predetermined temperature and pressure. For example, quantitative and qualitative analyzers such as refractometers, polarimeters and chromatographs are attached to the outlet line of the chamber and used to quantitatively detect and / or detect one or more components in the elution stream leaving the adsorbent chamber. Or it can be determined qualitatively. During the pulse test, the adsorbent is first filled to equilibrium with a specific desorbent by passing the desorbent through the adsorbent chamber. A small volume or pulsed feed mixture (which may be diluted with a desorbent) is infused over a period of 1 minute or more by switching the desorbent flow to the feed sample loop at zero time. The desorbent flow is resumed and the feedstock mixture components are eluted, for example, by a liquid-solid chromatographic operation. The eluate may be analyzed on-stream, or the eluate sample may be collected periodically and analyzed separately (off-line) to elute the envelope trace of the corresponding component peak. You may plot from the viewpoint of the component density | concentration with respect to the quantity of a thing.

  Using information obtained from pulse testing, adsorbent void volume, retention volume for the extracted or raffinate component, selectivity for one component to the other, stage time, resolution between these components, and desorption The rate of desorption of the extracted component by the agent can be determined. The retention volume of the extraction component or raffinate component can be determined from the distance between the center of the peak envelope of the extraction component or raffinate component and the peak envelope of the tracer component or some other known reference point. The volume is expressed in cubic centimeters of desorbent delivered during a time interval corresponding to the distance between peak envelopes.

  The retention volume of a good candidate system falls within the range set by extrapolation to a commercial design. A very small retention volume indicates that there is little separation between the two components (ie, one component is not adsorbed sufficiently strongly). A large extract component retention volume indicates that the desorbent is difficult to remove retained extract components.

  Conventional equipment for use in a fluid-solid contacting stationary bed may be used in the above described adsorption separation process incorporating a binder-free adsorbent comprising zeolite X with a small crystallite size. The adsorbent may be used in the form of a single fixed bed that alternately contacts the feed and desorbent streams. Thus, the adsorbent may be used in a single stationary bed that is alternately subjected to an adsorption step and a desorption step in a discontinuous (eg, batch) process. A swing floor type operation in which a plurality of floors are periodically used for a predetermined operation or process is also possible. Alternatively, two or more stationary bed sets may be used with appropriate piping / valves to allow continuous passage of the feed stream into any one of a number of adsorbent beds, Meanwhile, the desorbent stream is passed through one or more other beds in the set. The feed stream and desorbent flow may be either upward or downward through the adsorbent.

  Counter-current moving bed mode of operation provides another possible mode of operation that can achieve a stationary concentration profile of the feed mixture components, thereby introducing feed and desorbent streams As well as continuous operation with fixed points of withdrawal of the extraction and raffinate streams. Countercurrent moving bed or simulated moving bed countercurrent flow systems have much greater separation efficiency than fixed adsorbent systems and are therefore very often used for commercial scale adsorption separations. In the simulated moving bed process, the adsorption and desorption is carried out continuously in a simulated moving bed process, whereby continuous extraction (extraction) and extraction and raffinate streams (both outlet streams) and feed and desorbent streams. Both continuous use (input) of (both inlet flow) is possible.

  The operating principle and process sequence of a simulated moving bed flow system are described in US 2,985,589, US 3,310,486, and US 4,385,993, which are related to their teachings on a simulated moving bed flow system. , Incorporated herein by reference. In such a system, the forward movement of multiple access points along the adsorbent chamber mimics the movement of adsorbent contained in one or more chambers (opposite of liquid access point movement). Typically, only four of the many (16-24 or more) access lines to the chamber (s) are used to carry the feed, desorbent, raffinate and extract streams. Active at any given time. Concurrent with this simulated movement (eg, upward movement) of the solid adsorbent is movement of fluid occupying the void volume of the adsorbent packed bed (eg, downward movement). This fluid (eg, liquid) flow circulation may be maintained using a pump. Because the active liquid access point moves through the cycle, i.e. through each adsorbent bed contained in one or more chambers, the chamber circulation pump typically provides different flow rates. A programmed flow controller may be provided to set and control these flow rates.

  These active access points efficiently divide the adsorbent chamber into separate zones, each with a different function. Typically, three separate operational zones exist to cause the process, but in some cases, a fourth operational zone, selected as desired, is used. The number of zones used in the following description of the simulated moving bed process corresponds to the number of zones described in US 3,392,113 and US 4,475,954, which are also those for simulated moving bed operation. The teachings of which are incorporated herein by reference.

  The adsorption zone (zone 1) is defined as the adsorbent located between the inlet feed stream and the outlet raffinate stream. In this zone, the feed mixture contacts the adsorbent, the extracted components are adsorbed and the raffinate stream is desorbed. The general flow through zone 1 is from a feed stream passing into the zone to a raffinate stream passing out of the zone. Also, the flow in this zone is usually considered to be in the downstream direction when going from the inlet feed stream to the outlet raffinate stream.

  Immediately upstream of the fluid flow in zone 1 is the purification zone (zone 2). A purification zone is defined as the adsorbent between the outlet extraction stream and the inlet feed stream. The basic operation in this zone is the replacement of any raffinate component conveyed into zone 2 from the non-selective void volume of the adsorbent and is adsorbed within the selective pore volume of the adsorbent. Or desorption of any raffinate component adsorbed on the surface of the adsorbent particles. Purification is accomplished by passing a portion of the extraction stream leaving zone 3 into zone 2 at its upstream boundary, the extraction outlet stream, to cause replacement of the raffinate component. The flow in zone 2 is in the downstream direction from the extraction outlet stream to the feed inlet stream. This material then merges with the feed stream and flows through zone 1.

  Immediately upstream of zone 2 with respect to the fluid flowing in zone 2 is the desorption zone (zone 3). The desorption zone is defined as the adsorbent between the inlet desorbent stream and the outlet extract stream. The function of the desorbent zone is that the desorbent passing through this zone replaces the extracted components adsorbed in the adsorbent upon prior contact with the feed mixture in zone 1 in the previous operating cycle. Is to make it possible. The flow of fluid in zone 3 is essentially in the same direction as the flow in zones 1 and 2.

  In some examples, a buffer zone (zone 4) selected as desired may be utilized. This zone, defined as the adsorbent between the outlet raffinate stream and the inlet desorbent stream, is located immediately upstream of the fluid flow into zone 3 when used. Zone 4 can be used to keep the amount of desorbent required for desorption constant, as this causes a portion of the raffinate stream removed from zone 1 to pass into zone 4 and desorbent. This is because the can be moved out of the zone and into the desorption zone. Zone 4 contains sufficient adsorbent so that raffinate components in the raffinate stream passing from zone 1 into zone 4 can be prevented from passing into zone 3, thereby leaving zone 3. Can prevent the extracted stream from being contaminated. If the fourth zone of operation is not utilized, the extraction outlet stream will not be contaminated if there is a discernable amount of raffinate component in the raffinate stream passing from zone 1 to zone 3. In order to be able to stop the direct flow from zone 1 to zone 3, the raffinate stream passed from zone 1 to zone 4 must be carefully monitored.

  The cyclic advancement of the input (feed and desorbent) and output (extraction and raffinate) streams through the fixed bed of adsorbent to provide a continuous process performed in a simulated moving bed mode is This can be accomplished by utilizing a manifold system in which the valves in the manifold are operated in a manner that causes switching of the input and output flows, thereby allowing fluid flow to the solid adsorbent. Supplied in a pseudo countercurrent style. Another type of operation that can mimic the counter-current flow of solid adsorbents involves the use of a rotating valve, where the input and output streams are respectively separated by the valve into the adsorbent chamber. Directed to one of a number of lines coupled to the input feed stream, output extract stream, input desorbent stream, and output raffinate stream entering or leaving the chamber. The position advances in the same direction along the adsorbent bed. Both the manifold arrangement and the rotary disc valve are known in the art. Several valve devices are described in detail in US 4,434,051. Rotary disc valves that can be used in this operation are described in US 3,040,777, US 4,632,149, US 4,614,204, and US 3,422,848. In these patents, rotary valves are disclosed that can achieve the appropriate advancement of various input and output streams from a fixed source without difficulty.

  In many instances, one operational zone of a simulated moving bed process contains a much greater amount of adsorbent than another operational zone. For example, in some operations, the buffer zone may contain a smaller amount of adsorbent compared to the adsorbent present in the adsorption zone and purification zone. As another example, in the case of using a desorbent that readily desorbs the extracted components from the adsorbent, the amount of adsorbent required during the desorption zone can be in the buffer zone, adsorption zone, purification zone, or all these zones. Relative to the amount of adsorbent required. Also, the adsorbent need not be located in a single chamber (ie, column or vessel) and often uses two adsorbent chambers (eg, provided with 12 access lines each). . Additional chambers are also contemplated.

  Typically, at least a portion of the output extract stream recovers a portion of the desorbent (eg, for recycling to an adsorptive separation process as a desorbent recycle stream) and (eg, a reduced amount of desorption). In order to produce a purified extract product stream (containing the agent), it is passed through a separation process, for example a fractionation column. Preferably, at least a portion of the output raffinate stream also contains another portion of the desorbent (eg, also for recycling to the adsorptive separation process) and (eg, also a reduced concentration of the desorbent). ) Pass through the separation process to recover the raffinate product stream. In large petrochemical units, essentially all desorbent is recovered for reuse. The design of the fractionation equipment for this recovery depends on the material being separated, the desorbent composition and the like.

  Another type of simulated moving bed flow system suitable for use in the adsorption separation process described above is the co-current, high efficiency simulated moving bed process described in US 4,402,832 and US 4,478,721, These are incorporated herein by reference for their teachings on this alternative mode of operation. If the manufacturer has access to a slightly lower purity product, this process has advantages in terms of major cost savings and energy efficiency.

  The scale of the adsorption separation unit for the purification of para-xylene can vary from pilot plant scale (see eg US 3,706,812) to commercial scale, and for production flow rates 1 It can range from as little as a few milliliters of time to hundreds of cubic meters per hour.

  Overall, aspects of the present invention are unique to binder-free adsorbents, including zeolite X, with small crystallite size, for use in adsorptive separations, and particularly for use in commercial simulated moving bed operations. It is aimed at the utilization of a special property. In view of the present disclosure, it will be appreciated that several advantages can be achieved and other advantageous results can be obtained. A person skilled in the art who has knowledge gained from the present disclosure can make various changes in the composition of the desorbent and the processes using these compositions without departing from the scope of the present disclosure. You will understand that it is possible. The chemical processes, mechanisms, modes of interaction, etc. used to explain theoretical or observed phenomena or results shall be construed as illustrative only, and the appended claims Is not to be construed as limiting in any way.

  The following examples are given as representative examples of the invention. These examples are not to be construed as limiting the scope of the invention, as these and other equivalent embodiments will become apparent in light of the present disclosure and the appended claims. To do.

Example 1
Synthesis of binder-free adsorbent particles containing zeolite X with small crystallite size:
A small crystallite size zeolite X powder made according to the procedure described herein is mixed with a known amount of commercially available kaolin and water to form an extrudable paste, which is then extruded. Composite pellets containing zeolite X with a small crystallite size of 50-90% in kaolin were formed. The composite was then dried at 100 ° C. and finally fired at> 600 ° C. to convert the binder kaolin to metakaolin. The binder metakaolin was then converted to zeolite X by hydrothermal treatment in <5 wt% NaOH solution with> 2 Na / Al (from metakaolin) with gentle agitation for up to 10 hours. . The kaolin clay had a molar ratio of silica to alumina of 2.0, but this ratio in the converted zeolite X is, for example, colloidal silica sol, silicic acid, alkali metal silicic acid during the caustic digestion of metakaolin. It has been found that this can be increased by adding a silica source such as salt, silica gel, or reactive particulate silica. The silica source could be added to the mixture to be formed, the alkaline reaction medium, or both. Thus, the resulting binder-free adsorbent particles had both a preparation portion and a conversion portion of zeolite X having a molar ratio of silica to alumina of 2.5. The preparation part of zeolite X had a small crystallite size. The adsorbent particles are used in the columns described herein to convert sodium-type zeolite X into barium / potassium-type having 99% or more ion exchange sites substituted by a combination of barium and potassium. Ion exchange.

Example 2
Selectivity test:
A conventional adsorbent comprising zeolite X with 6% by weight LOI exchanged with barium and potassium was evaluated by adsorptive separation of para-xylene. The standard pulse test described above was performed by first packing the adsorbent into a 70 cc column under the desorbent paradiethylbenzene. A feedstock pulse containing an equal amount of ethylbenzene and each of the three xylene isomers was injected with a linear nonane (n-C9) tracer. Pulse testing was performed at various column temperatures ranging from 121 ° C. to 177 ° C. (250 ° F. to 350 ° F.) to examine the effect of temperature on selectivity. Paraxylene selectivity was determined from the component peaks obtained from each of these pulse tests. The result is shown in FIG.

  The results show that paraxylene / metaxylene selectivity “P / M” and paraxylene / orthoxylene selectivity “P / O” increase at lower adsorption temperatures, while paraxylene / ethylbenzene selection The property “P / E” means that it hardly varies with temperature. This demonstrates that a lower adsorption separation temperature is preferred from a thermodynamic equilibrium standpoint (although not necessarily from a kinetic standpoint).

Example 3
Capacity test:
Chromatographic separation (dynamic capacity or breakthrough test) was performed using the conventional adsorbent described in Example 2. A column containing 70 cc of the adsorbent initially packed under paradiethylbenzene was packed with a sample feed mixture containing orthoxylene, metaxylene, paraxylene, and ethylbenzene. Breakthrough tests at various column temperatures ranging from 121 ° C. to 177 ° C. (250 ° F. to 350 ° F.) and the effect of temperature on adsorbent capacity and paraxylene / paradiethylbenzene selectivity “PX / pDEB Sel” Inspected. The result is shown in FIG. This result further confirms that a lower adsorption separation temperature is desirable in terms of total adsorbent capacity so long as it does not adversely affect paraxylene / desorbent selectivity.

Example 4
Mass transfer rate:
Using the data obtained from the pulse test described in Example 2, “DW” or “Delta W”, ie, the half width of the paraxylene peak (ie, the peak envelope width at half strength), the linear nonane The half width of the tracer peak minus the variance was determined as a function of temperature. The result is shown in FIG. This result shows that mass transfer rate limiting becomes significant at a temperature lower than 177 ° C. (350 ° F.) in the conventional para-xylene adsorption separation process.

Example 5
Adsorption separation of para-xylene in simulated moving bed system, temperature effect:
The performance of the conventional adsorbent described in Example 2 in the adsorption separation of paraxylene in a pilot plant operating in a simulated moving bed system will be described with reference to FIG. In all conditions analyzed, the total para-xylene recovered in the extract stream was 95% by weight. Performance parameters measured at 150 ° C. (302 ° F.) (solid square symbol) are shown as a function of process cycle time compared to performance parameters measured at 177 ° C. (350 ° F.) (solid triangle symbol). . In all cases, the zeta value at the lower temperature (Y axis, left side) was lower than the corresponding value at the higher temperature. This is due to improved paraxylene adsorption selectivity and adsorbent capacity, as confirmed in the pulse and breakthrough tests described in Examples 2 and 3, at higher operating temperatures at higher paraxylene. It shows that productivity has been achieved.

  The zeta value is a measure of the selective pore volume required to process the unit volumetric rate of the paraxylene-containing feedstock. Thus, lower zeta values are directional, indicating higher throughput or productivity. In particular, the zeta value is calculated as the product of A / Fa and θ, where A is the speed of the selective circulation of the adsorbent pore volume, and Fa is the C8 alkyl aromatic. Is the volumetric rate of the feed stream containing the hydrocarbon mixture, and θ is the cycle time. The quantity A / Fa is shown in FIG. 5 for each combination of cycle time and temperature tested. The near convergence at low cycle times for curves produced by high and low temperature operations shows low temperature performance as cycle time decreases (and as productivity or zeta is maximized). This means that the conspicuousness of the benefits is reduced.

  This actually observed barrier to obtaining even higher productivity is due to mass transfer rate limiting. The lower (white diamond) curve in FIG. 5 shows the possible feed flow obtained by reducing the operating temperature from 177 ° C. (350 ° F.) to 150 ° C. (302 ° F.) at each cycle time. From the viewpoint of increasing the volumetric measurement speed (Y axis, right side), the effect of mass transfer rate limiting on productivity will be described. With a 34 minute cycle time, for example, the advantage of lower operating temperature translates to a 14% increase in volumetric rate at which the C8 alkyl aromatic feed can be processed. However, the productivity advantage is reduced to only 7% as a result of mass transfer rate limiting at a cycle time of 24 minutes. On the other hand, the broken line in FIG. 5 explains the advantages of the paraxylene adsorption separation performance parameter and the feedstock productivity that are theoretically possible in the absence of mass transfer rate limiting.

  Thus, laboratory and pilot plant scale studies described in Examples 2-5 show that adsorbent volume, adsorption selectivity, and increased liquid density per unit volume of adsorbent due to lower operating temperatures. This explains that the material transfer rate is reduced while a significant advantage is produced in the process. As a result, in the adsorptive separation of para-xylene from relatively impure C8 alkyl aromatic hydrocarbon mixtures, the thermodynamically preferred at temperatures ranging from 130 ° C. (270 ° F.) to 165 ° C. (330 ° F.). It is difficult to make full use of the operating regime. As shown in FIG. 5, as the cycle time is reduced, the productivity advantage of 150 ° C. (300 ° F.) operation decreases with conventional adsorbents.

Example 6
Adsorption and separation of para-xylene in simulated moving bed system, bed concentration profile:
Desorbent concentrations for each of the 24 adsorbent beds obtained during the adsorption separation of paraxylene in a pilot plant operating in a simulated moving bed system using conventional adsorbents (as described in Example 2). Profiles and feedstock mix components were evaluated. A liquid composition “investigation” (ie, analysis of the liquid composition in each bed) was performed during steady state operation at 150 ° C. (302 ° F.) and with 95% paraxylene recovery in the extraction stream. . Concentration profiles were obtained using 34 and 24 minute cycle times, respectively. These findings again reassured that at this temperature, significant mass transfer rate limiting hindered better realization of more favorable paraxylene adsorption selectivity and adsorbent capacity. These adverse effects due to the rate limiting of mass transfer are manifested at the more dispersed paraxylene front in the adsorption zone (zone I above). The theoretical productivity increase associated with low temperature operation cannot be achieved using conventional adsorbents because the cycle time is reduced to more commercially desirable values (eg, 24-34 minutes).

  In a micrograph of a binder-free adsorbent containing zeolite X by high-resolution scanning electron microscopy (HR SEM), the zeolite X crystallites provide a major diffusion path that limits the mass transfer rate. It was shown that.

Example 7
Improve adsorption separation of para-xylene in simulated moving bed mode by reducing zeolite X crystallite size in adsorbent:
FIG. 6 compares the performance of a conventional adsorbent containing zeolite X having an average crystallite size of 1.8-2.0 microns versus an adsorbent containing zeolite X having an average crystallite size of 1.4 microns. To do. The zeolite X crystallite size distribution for a representative conventional zeolite X sample and another sample having a reduced crystallite size (such as that used in the test adsorbent) is shown in FIG. Here, the average crystallite size of the conventional zeolite X is 1.8 to 2.0 microns. Adsorbents containing zeolite X with reduced crystallite size further improve overall paraxylene recovery while maintaining all other conditions in the paraxylene adsorption separation process operating in simulated moving bed mode. It became possible to raise it. For the reasons discussed above, and as shown in FIG. 6, these benefits associated with reducing the zeolite X crystallite size as cycle time is reduced, and in particular, this attendant improvement. Mass transfer became more prominent.

  Therefore, the mass transfer path was shortened by reducing the size of the zeolite X crystallites in the adsorbent, and as a result, the mass transfer rate was improved. Paraxylene manufacturers are striving to improve productivity in adsorbent separation processes operating in simulated moving bed mode by reducing cycle times and making them less than the standard 34 minute operating parameters. This has important commercial implications. For example, many commercial operations are performed using a 30 minute cycle time or even a 27 minute cycle time.

  In these cases, the use of an adsorbent comprising zeolite X with a small crystallite size is particularly advantageous to overcome the mass transfer rate limitations observed for operating temperatures below 175 ° C. (350 ° F.). . Zeolite X with a small crystallite size is synthesized with an average crystallite size in the range of 0.5 microns (500 nanometers) to 1.4 microns, which allows the zeolite crystallite size to be used for a given application. Optimization is possible, resulting in adsorbent formulation and whole process.

Example 8
Selectivity and capacity of binder-free adsorbents, temperature effects:
FIG. 8 shows para-xylene / at different adsorbent hydration levels (or different LOI values, as described above) at 150 ° C. (302 ° F.) compared to 177 ° C. (350 ° F.). Describe the selectivity benefits of meta-xylene (“P / M”), para-xylene / ortho-xylene (“P / O”), and para-xylene / ethylbenzene (“P / E” or “P / E dyn”) Is. Selectivity values P / M (square), P / O (triangle), and P / E (diamond) were obtained by standard pulse tests as described in Example 2. On the other hand, the selectivity value P / E dyn (asterisk) was obtained by the dynamic or breakthrough test described in Example 3. FIG. 9 illustrates the overall adsorbent capacity measured during these tests. In both FIG. 8 and FIG. 9, the data points obtained at the lower temperature are connected by a solid line, while the data points obtained at the higher temperature are connected by a broken line.

  The data in FIGS. 8 and 9 further illustrate the advantages of lower temperature operation in this case using a binder-free adsorbent. The para-xylene selectivity for other xylene isomers and ethylbenzene is based on the operating or adsorption temperature based on the results of both pulse and breakthrough tests at 150 ° C. (302 ° F.) and 177 ° C. (350 ° F.). Can be significantly increased (for example, about 15% to 20%). Furthermore, the adsorbent capacity increases in operation at lower temperatures. These advantages associated with lower temperature operation include improved mass transfer and more associated with the binder-free adsorbent described herein, including at least a portion of zeolite X having a small crystallite size. As a result of the high capacity, it can be more fully utilized.

Claims (10)

  A method for separating para-xylene from a mixture comprising at least one other C8 alkyl aromatic hydrocarbon, said mixture having an average crystallite size of less than 1.8 microns under adsorption conditions Contacting with a binder-free adsorbent comprising zeolite X.   The method of claim 1, wherein the average crystallite size is from 500 nanometers to 1.5 microns.   3. A process according to claim 1 or 2, wherein the binder-free adsorbent comprises a converted portion of zeolite X resulting from the conversion of the zeolite X precursor.   The process according to claim 3, wherein the conversion part of the zeolite X is present in an amount of 5% to 40% by weight relative to the adsorbent.   The process according to claim 3 or 4, wherein the conversion part of the zeolite X has a molar ratio of silica to alumina of 2.1 to 3.0. The mixture includes ortho-xylene, meta-xylene, para-xylene, and ethylbenzene, and by contacting the mixture with the adsorbent, the xylene, meta-xylene, and ethylbenzene present in the non-adsorbing phase are preferred. A process in which adsorption of para-xylene present in the adsorption phase occurs,
Washing away the non-adsorbed phase from contact with the adsorbent, and desorbing paraxylene in the adsorbed phase from the adsorbent;
Further including
Paraxylene in the adsorbed phase is desorbed into an extract stream containing a desorbent, and the non-adsorbed phase is washed out into a raffinate stream containing a desorbent, where the desorbent is toluene. The method according to claim 1, comprising an aromatic ring-containing compound selected from the group consisting of benzene, indane, paradiethylbenzene, 1,4-diisopropylbenzene, and mixtures thereof. Performed in a simulated moving floor system,
A feed stream and a desorbent stream are packed into the adsorbent bed, the feed stream comprises a mixture comprising ortho-xylene, meta-xylene, para-xylene, and ethylbenzene, and the desorbent stream comprises a desorbent. Including;
An extract stream and a raffinate stream are removed from the bed of the adsorbent; and having a cycle time of less than 34 minutes;
The method of claim 6. 2.1 to 3.0 of _SiO 2 / _Al 2 _{O 3} molar ratio and 500 a first portion of zeolite X having an average crystallite size of nanometers to 1.5 microns, and SiO ₂ of 2.1 to 3.0 A second portion of zeolite X having a / Al ₂ O ₃ molar ratio and an average crystallite size of less than 500 nanometers,
Including an adsorbent.   The adsorbent of claim 8, wherein the second portion is present in the adsorbent in an amount of 5 wt% to 40 wt%, and the adsorbent is binder free. A method for preparing a binder-free adsorbent having improved mass transfer properties comprising:
(A) forming particles comprising zeolite X and zeolite X precursor having an average crystallite size of 500 nanometers to 1.5 microns;
(B) activating the zeolite X precursor of the particles formed in step (a) at a temperature of 500 ° C. to 700 ° C., and (c) the zeolite X precursor activated in step (b). Digesting the particles with a caustic solution to obtain a binder-free adsorbent;
Wherein step (c) is carried out in the presence of a silica source.

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